1. Field of the Invention
The present invention relates to an optical fiber laser incorporating an erbium doped fiber as a gain medium. In particular, it relates to an optical laser that emits laser light in the wavelength band of 2.8 μm (2.7 μm to 3.0 μm) and to a method of emitting the laser light.
This application claims priority on Japanese Patent Application No. 2003-156988 filed on Jun. 2, 2003, Japanese Patent Application No. 2003-165075 filed on Jun. 10, 2003 and Japanese Patent Application No. 2003-382617 filed on Nov. 12, 2003, of which are incorporated herein.
2. Description of the Related Art
Lasers operating in the wavelength range of 2.8 μm to 2.9 μm, which is the water absorption band, are used in the medical field and the like. As such a laser operating in the wavelength range of 2.8 μm to 2.9 μm, there has been proposed an optical fiber laser incorporating an erbium doped fiber (also referred to as EDF, hereinafter) as a gain medium (for example, see Applied Physics B by M. Pollnau et al., 1998, Vol. 67, pp. 23–28). In general, the optical fiber emits laser light in the wavelength band of 2.8 μm (2.7 μm to 3.0 μm) using pumping light in a wavelength of shorter than 980 nm.
In order to increase the power of the laser light of the optical fiber laser described above, the EDF serving as the gain medium must be longer. However, the longer EDF leads to a longer resonator (or a longer resonator length), resulting in a reduced peak power of the laser light and a larger temporal half-width of the laser light.
Thus, there is a need to enhance the emission efficiency of laser light to achieve a high laser power with a relatively short resonator length.
FIG. 7 is an energy level diagram for erbium ions of an erbium doped fiber.
When pumping light in a wavelength band of 980 nm is launched into the EDF, ions in the ground state (4I15/2) are excited to the upper level (4I11/2) due to ground state absorption (also referred to as GSA, hereinafter). Then, when the ions decay to the lower level (4I13/2) from the upper level (4I11/2), the laser light in a wavelength band of 2.8 μm (2.7 μm to 3.0 μm) is emitted.
If excited state absorption (also referred to as ESA, hereinafter) occurs in the upper level (4I11/2), the ions are excited to a still higher energy level (4F7/2), and the emission efficiency of the laser light in a wavelength band of 2.8 μm (2.7 μm to 3.0 μm) is reduced.
Recently, there has been a report on absorption spectra of GSA and ESA in a wavelength band of 980 nm for the EDF (for example, see the “Proceedings of the Meeting: Fiber laser sources and amplifiers” by Richard Quimby, The International Society for Optical Engineering, 1991, vol. 1581, pp. 72–79).
If the ESA is reduced, a higher emission efficiency can be achieved, and the laser power of the optical fiber can be raised without elongation of the optical fiber. However, research concerning the excitation condition has been insufficient, and the optimum wavelength of the pumping light has not been found yet.
FIG. 8 is a graph showing a relationship between the peak wavelength of laser light emitted from a conventional optical fiber laser and the power of the pumping light.
The conventional optical fiber laser uses one pumping light in the wavelength of shorter than 980 nm. As can be seen from the drawing, as the power of the pumping light increases, the peak wavelength of the emitted laser light is shifted toward longer wavelengths. Thus, the conventional optical fiber laser has a problem that the peak wavelength of the laser light varies with the power of the pumping light when adjusting the power of the pumping light to achieve a desired value of the laser light power. For example, in the case in which the optical fiber laser is used for medical purposes, if the peak wavelength of the laser light varies, there arises a problem of a reduced capability of cutting or ablating of living tissues.
The mechanism of how this problem occurs will be described below.
In general, each energy level at the time of laser emission is often represented as one line as shown in FIG. 9. However, actually, each level splits into several sub-levels at narrow intervals as shown in FIG. 10. These sub-levels are referred to as Stark levels. The wavelength of the emitted laser light (emitted light wavelength) depends on the Stark levels between which the ion transition causing the light emission occurs.
If the intensity (power) of the pumping light is increased, ions at the ground state level (4I15/2) shown in FIG. 9 are excited. Therefore the number of ions occupying the upper level of erbium (4I11/2) and the lower level of erbium (4I13/2) increase.
Since lower Stark levels are occupied by ions earlier than higher ones in each energy level, the lower Stark levels in the lower level (4I13/2) are also occupied by ions earlier than the higher ones.
As the intensity (power) of the pumping light increases, the lower Stark levels of the lower level (4I13/2) are occupied by ions and become incapable of contributing to light emission, and thus, the higher Stark levels, which are occupied by fewer ions, participate in light emission. As a result, the interval between the lower level (4I13/2) and the upper level (4I11/2) is reduced, and thus, the wavelength of the laser light (emitted light wavelength) is disadvantageously shifted toward longer wavelengths.